A convection oven heats an object in an oven cavity by transferring heat energy from heating elements to the object by circulation of a gas within the oven cavity. Typically, a thermal sensor senses the temperature of the gas and a regulator controls the operation of the heating elements in response to the sensed temperature to maintain a desired operating temperature in the oven cavity. Although the circulated gas in a convection oven for cooking food is typically air, other gases may be employed such as nitrogen, steam, or combustion gases from gas-fired burners, depending upon the oven application. Thus, although convection ovens are commonly used for cooking and baking food, convection oven applications are not limited to cooking and baking. Convection ovens may also be employed in industrial or commercial applications that do not directly cook food.
In a standard oven, the oven cavity temperature is controlled by a temperature regulator that turns a heating element on or off as necessary. Convection oven heating elements typically consist of either a gas-fired combustion chamber separate from the oven cavity, or a resistive heating element energized by an electric current, but may also include other types of heating elements such as, for example, an infrared energy source.
A major problem in convection ovens used for cooking food has been obtaining uniform heating of the food products in the oven. This problem is aggravated when food is placed on cooking racks at multiple elevations within the oven compartment. Maintaining high food quality requires even and thorough cooking of food throughout the oven cavity. Minimizing cooking time strongly depends upon the distribution of hot air throughout the oven cavity during cooking. The distribution of hot air is strongly impacted, for example, by opening the oven door. Thus, because airflow is such an important factor in achieving uniform air temperature distribution, managing the airflow in the oven cavity is the key to improving both the quality of cooked food and the time required to cook the food in a convection oven.
It is well understood that using a blower, such as a fan, to promote air circulation can dramatically improve the uniformity of air temperature distribution within the oven cavity of a convection oven. However, unmanaged air flow can still be uneven, leading to undesirable drying of foods, causing batters to lean, and over-browning the edges of bakery items such as cakes and muffins.
Convection ovens typically employ one of three types of air circulation arrangements in combination with conventional resistive heating elements. Each type of air circulation arrangement provides a different degree of control over air temperature distribution in the oven cavity. The first type of air circulation arrangement, passive circulation, takes advantage of naturally rising convection currents within the oven cavity. Such a passive arrangement has no ability to manage airflow, however. The second type of air circulation arrangement, as described, for example in U.S. Pat. No. 4,071,739, employs an unheated blower to force air to circulate in the oven cavity. Because the heat source and the blower are physically separate, this system provides limited control over air temperature distribution. The third type of air circulation arrangement forces air into the oven cavity after heating the air by a heating element positioned adjacent to the blower. This third type permits the best management of hot air temperature distribution in the oven cavity.
Several methods of improving food quality and reducing cooking time using forced hot air circulation directly to cook the food are known. One such method is disclosed in U.S. Pat. No. 4,308,853. In this method, a blower forces hot air through a system of mechanical passageways that guide the hot air to food located in a series of vertically stacked compartments. Two zones of airflow are established in each compartment: a laminar flow zone heats the bottom of the food and a turbulent flow zone heats the top of the food. Such an oven is limited, however, to food that fits into fixed-height compartments. This patent also describes forcing air from one side of the oven to an intake on the opposite side of the oven thereby creating substantially linear airflow through the oven cavity. This arrangement requires rotating the food around the vertical axis. Another previous attempt to improve airflow management in a convection oven using forced circulation has employed a baffle with exhaust openings on the top and bottom as well as the sides thereof. Such a baffle is used to direct the airflow from a blower, resulting in a “toroidal” airflow in which the centrally located blower intakes air omni-directionally from an empty cavity. See, for example, U.S. Pat. No. 3,797,473. However, when food is placed in such an oven, the airflow can be significantly blocked, particularly in the non-horizontal plane, resulting in turbulence and reduction in airflow effectiveness. Furthermore, the vertically circulating currents can experience divergent temperatures due to passive convection, leading to non-uniform temperature distribution. Other attempts to improve forced hot air convection involve fixing jets of hot air around the food (see U.S. Pat. No. 4,951,645), rotating the food itself (e.g., a rotisserie, see also U.S. Pat. No. 5,485,780), or rotating the hot air source (see U.S. Pat. No. 4,503,760). Each of these approaches has complexity, space, and/or cost drawbacks.
The time for the air temperature distribution to recover after an oven door is opened and re-closed is an important factor in determining cooking time. Because opening the oven door dramatically disrupts the hot air temperature distribution in the oven cavity, a forced air system shortens the recovery time and thereby improves overall cooking time. However, the improvement in recovery time in current convection ovens is limited by the airflow capacity that a single blower can provide to the oven cavity.